Although one class of lasers is frequently the basis for communications, ranging, and various types of instrumentation, a second class of lasers is widely used to deliver high levels of energy per unit area to materials of various types for a variety of applications including drilling, cutting, welding, scribing, marking, processing and medical treatment. The wavelengths of the laser sources so used commonly range from 193 nanometers in the vacuum ultraviolet to 10.6 microns in the infrared.
High energy lasers of this class are used in conjunction with optical systems which accept the laser beam and modify it to perform a specific function; such systems concentrate the laser energy into a small spot focused on the workpiece, or object, to be treated. The spot size, depending upon the specific laser action desired may vary from a few microns to a millimeter in diameter or larger.
Among the objects to be treated with lasers delivering high energy levels per unit area include biological tissues (for treatment or micro surgery), gemstones (for cutting or for identification marking) and microcircuit boards (for exposing circuit components under a polymer or other overcoat).
The lasers used for the treatment of microcircuit wafers are preferably those of the pulsed type and having very short wavelengths such as are provided by argon fluoride lasers operating in the ultraviolet region at 193 nanometers. The optical system for delivering the 193 nm energy requires the use of quartz lenses for efficient refraction, and multi-layer mirrors designed for efficient reflection of this energy.
In those applications where the laser spot size is large as in the case of welding using 10.6 micron carbon dioxide lasers the effect of the laser action on the workpiece is readily apparent under low magnification and even to the unaided eye. When the spot diameter is in the range of a few microns to a millimeter, however, the fine details in the altered workpiece after it has been imprinted with the laser spot can be distinguished only under high magnification and high resolution. Because it is frequently necessary to provide access to the workpiece surface for purposes of manipulation following modification with laser energy, a relatively large clearance is desirable between the viewing optics and the work surface. The magnification required for viewing is ordinarily about 1000.times., with viewing commonly done on a video screen. The physical clearance obtained with a moderate power, fairly high numerical aperture, long working-distance microscope objective which would be used for forming an image of the imprint made on the workpiece, is about 8 to 10 mm.
In an ideal system the active area on the workpiece would be continuously viewable during laser action, and this is achievable in an arrangement using a beamsplitter, provided (a) the wavelength of the laser radiation falls within or close to the visible spectrum, out to about 0.8 micron in the near infra red and 0.35 microns in the near ultraviolet, or (b) the viewing is also with visible laser light, thereby reducing some of the problems of achromatism. With condition (a) it may be possible to design an objective lens which would be sufficiently well corrected to act both as the lens for focusing laser energy on the workpiece and as the lens for imaging the active area photographically or onto a video camera. With condition (b) the optical design area of interest on the target could not be seen in true color, and there would remain the problem of laser speckle.
With the white light system of (a) it is not possible to design such a system if the laser wavelength falls far outside the visible spectrum. If the laser source is in the fairly far ultraviolet spectrum (193 nanometers or 0.193 microns) or the fairly far infra red spectrum (10.6 microns) the required optical corrections for sharp imagery in both the visible spectrum (0.4 to 0.7 microns) and the laser spectrum is not achievable in a lens or mirror objective having a large relative aperture and long working distance. In addition, a beam splitter which would transmit the laser light and reflect the visible light, or vice versa, would be costly and not necessarily efficient.
Related devices appear in the prior art. U.S. Pat. No. 4,584,455 describes a laser beam machining apparatus for trimming thick film resistors and the like which employs a first working laser for removing material from the workpiece and a second visible light laser for viewing the workpiece, the two lasers having a coincidence via beam splitting mirrors. A scanning system having a stated resolving power of ten microns positions the laser spots on a selected area of the workpiece. The area on the workpiece illuminated by the visible laser is received on a CCD pickup camera, the output of which, through coordinate and compensation circuits, corrects for temperature drift. Although values of focal length or relative aperture of the scanning lens or CCD camera lens, or of distances between the various optical components are not given in the specification, an examination of FIG. 5 indicates that the scanning lens might have an aperture of f/8. The lens on the CCD camera, even if an extreme telephoto having an effective focal length twice the physical length shown in FIG. 5, would produce on the CCD an image of the visible laser spot on the workpiece at a magnification of about 0.3. Thus a 10 micron spot on the workpiece would be imaged as a 3 micron spot on the CCD. If the CCD camera has 512.times.512 elements along a 10 mm side it can resolve 50 lines/mm or a 20 micron diameter spot; with the conjugates shown in FIG. 5, the smallest image element it can resolve at the workpiece is 20.times.3.3 or 66 microns. If the lens on the CCD camera has a focal length corresponding to an ordinary lens having the relative size shown in FIG. 5, the finest detail visible in the workpiece will be 100 microns in size or greater.
U.S. Pat. No. 4,532,402 shows an apparatus for repairing integrated circuits by positioning a laser beam on a semiconductor surface through use of galvanometer driven mirrors. The integrated circuit board or workpiece is held on x and y axis platforms which position the workpiece under an optical system which includes a microscope objective and the galvanometer driven mirrors. Positioning of the platforms is achieved through interferometry and a control processor. There is no provision for an optical system for viewing the workpiece.